The Impact of Rising Inequality on Health at Birth
Anna Aizer Brown University and NBER
Florencia Borrescio Higa Brown University
Hernan Winkler World Bank
May 2013
Abstract: The income distribution has widened significantly over the past 40 years, primarily due to skill biased technological change which has disproportionately raised the income of the most highly skilled. A major concern over rising inequality is its potential to reduce intergenerational mobility, leading to even greater inequality in the next generation. We estimate the impact of rising inequality over the period 1970-2000 on offspring health at birth, a measure of human capital that has been shown to be highly correlated with future education, IQ and income. We define inequality three ways: as a group-level measure (the Gini coefficient for each county), as an individual-level measure of relative deprivation and as an ordinal measure of rank. We find that including a modest set of controls reduces the negative relationship between aggregate measures of inequality and health, and limiting variation to changes in inequality over time within an area or instrumenting for inequality eliminates it completely. However, this null result likely reflects heterogeneity in the effect of rising inequality. When we estimate the impact of relative deprivation or rank on newborn health, we find negative and significant effects. Together these results suggest that increases in inequality in the current generation may lead to reduced intergenerational mobility and greater levels of inequality in the next generation.
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I. Introduction
Income inequality has been on the rise in most industrialized nations since the 1970s. In the
US, for example, the Gini coefficient increased steadily from .39 in 1970 to .47 by 2010. There has been considerable discussion of the causes of the rise in income inequality. Most research based on developed countries points to the increase in skill-biased technological change and globalization as the most important factors.1
In this paper we consider one potential consequence of rising inequality. Specifically, we
estimate the impact of inequality on health at birth – a measure of the initial human capital of the
next generation. We focus on health at birth for multiple reasons. First, newborn health is
sensitive to changes in short term conditions (eg, Almond, 2006). This makes it easier to isolate
the economic conditions affecting health. Second, health at birth has been shown to be an
important determinant of long term outcomes such as educational attainment, IQ and earnings
(Black, Devereaux and Salvanes, 2007). Third, individual-level data on birth outcomes has been
collected and reported consistently at a local geographic level (county) for the period 1970-2010,
allowing one to estimate the impact of increases in inequality and relative income at a local level on individual outcomes. Finally, by examining the impact of inequality on newborn health, we can learn not only about how inequality affects health, but how it might affect intergenerational mobility and inequality of the next generation.
In our estimates of the impact of rising inequality on health at birth, we define inequality in two ways. First, we define it as the Gini coefficient for the local area (state or county). This measure is common to all individuals in the area. Unlike much of the existing empirical work that relies on cross sectional variation, we utilize a 30 year panel of data which allow us to
1 Between 1979 and 2002, the causal return to education increased by 40 percent (Deschenes, 2006).
2 include area fixed effects, thereby limiting variation to that within an area over time and reducing potential omitted variable bias. Another contribution is that we instrument for inequality
(Boustan, Ferreira, Winkler and Zolt, 2012). The instrument allows us to isolate the change in the local income distribution that is driven by national shifts in the income distribution over time, not changes in the underlying composition of the area. More specifically, we construct an instrument for local, (state or county-level), distribution of income by holding local area income fixed at the 1970 distribution and match this initial distribution to national patterns in income growth for different points in the distribution of income.2
In initial results using aggregated data, we find a negative relationship between the Gini coefficient and newborn health (birth weight and an indicator for low birth weight). However, as we include even a parsimonious set of controls, the effect declines in magnitude and when we include area fixed effects thereby limiting variation to that within an area over time and/or instrument for the Gini, the estimated effect is neither large nor significant. When we repeat the analysis with individual level data that includes maternal income, we find the same pattern. This is consistent with either no causal effect of inequality on health, or significant heterogeneity in the effect that, in the aggregate, leads to no observed effect.
To help interpret this effect, we turn to two individual level measures of relative income.
The first is the Yitzhaki measure of relative deprivation and it reflects the average distance
2 For example, consider two counties, A and B. In 1970, county A had a disproportionate share of women in the bottom quartile of the (national) education distribution at the time, while county B had a disproportionate share of women in the top quartile of the (national) education distribution. By 1980, the distribution of education county A had changed so that it is more similar to the distribution of education in county B. However the instrument for the distribution of income in county A is calculated by holding the distribution of education fixed at the 1970 level and then predicting the income distribution based on national trends in income growth for the initial distribution of education.
3 between an individual’s own income and the income of those above her. When we estimate the impact of relative deprivation on newborn health including own absolute income, area fixed effects and instrumenting for relative deprivation, we find that the one’s relative deprivation is negatively related to newborn health. The second measure of relative income is an ordinal measure: rank based on the income of new mothers in the state. We find that conditional on absolute income, rank is related to newborn health and that the effect is non-linear (with greater effects for those at the bottom) and greater in areas characterized by larger income dispersion.
These findings suggest that for the poor, rising inequality reduces the initial human capital of the next generation, thereby reducing intergenerational mobility for the most disadvantaged.
II. Background
A. Inequality and the Intergenerational Transmission of Economic Status
There is a considerable theoretical literature on the relationship between income inequality and intergenerational mobility, largely in the macroeconomics literature (see Piketty, 1998 for a review). In general, the research suggests that greater inequality should reduce intergenerational mobility and growth. There are a number of mechanisms. The first has to do with imperfect credit markets and investments in human capital. Galor and Moav(2004) posit that because of credit constraints, in an unequal society there will be suboptimal investment in the human capital of the next generation (see Burtless and Jenckes, 2003, for a more microeconomic perspective).
Not only will this lead to greater inequality in the next generation, but also to reduced overall growth. A second potential mechanism relates to segregation. Durlauf (1996) argues that greater inequality will lead to greater segregation by income which will have the effect of reducing the
4 human and social capital of the next generation. Finally, models of statistical discrimination can also explain how greater inequality in one generation will lead to reduced mobility and increased inequality in the next generation. In the presence of both inequality statistical discrimination, if employers discriminate in their hiring of the relatively disadvantaged, this will lead to reduced human capital investments among the discriminated group (the disadvantaged) and even greater inequality in the next generation (Arrow, 1973, Piketty, 1998).
While there is considerable theoretical work on this topic, the empirical work characterizing the relationship between inequality and intergenerational mobility is relatively under-developed. Corak (2012) shows that across OECD countries, there is a strong correlation between measures of inequality (the Gini) and intergenerational transmission of earnings. While the evidence he presents is suggestive, questions remain. First is the question of whether and to what extent this relationship exists at a more micro level. Second is the question of what underlies this relationship and whether it can be characterized as causal.
In this paper we attempt to shed greater light on this by estimating whether inequality affects the initial human capital of the next generation – thereby providing a mechanism by which inequality of one generation may lead to lower intergenerational mobility. We do so in the context of newborn health. While there does not yet appear to be any empirical analysis looking at this specific question, there is a substantial empirical literature looking at inequality and health more generally which we review below.
B. Inequality, Relative Income and Health
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Inequality and relative income or relative deprivation are closely related but distinct concepts. Areas characterized by greater inequality have higher relative deprivation. However, inequality characterizes an entire group of individuals, whereas relative income or deprivation is specific to an individual within a group.
There are three reasons why inequality could be related to health. The first has to do with non-linearities in the production of health. If maternal income produces child health (see Case,
Lubotsky and Paxson, 2002), but is marginally more productive at low levels of income than at high levels, then an increase in inequality would reduce average health. This could explain the link between income inequality and mortality that is observed in aggregate data at the country, state or metropolitan level. Miller (2002) presents evidence that non-linearities in the relationship between income and health can explain much of the observed relationship between the Gini and health in the context of adult mortality. We refer to this channel as an “indirect” effect.
The second has to do with relative status and the higher levels of psychological stress that characterize those who are relatively worse off than their peers. In this formulation, relative deprivation may be measured in terms of income, education, social or political status, all of which are correlated. Individuals who feel greater stress because of their lower status may be stressed and/or depressed and also more likely to engage in behaviors that negatively affect health (poor eating and exercise habits, smoking, etc). In previous work, Aizer, Stroud and Buka
(2012) found that the stress hormone cortisol is higher in poor women and that higher levels of cortisol during the prenatal period is associated with worse outcomes for the offspring (lower IQ, worse health and less completed schooling). These data, however, did not allow for the examination of the direct role or impact of inequality on cortisol levels and offspring outcomes.
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Much of the evidence regarding a causal impact of social status on health is drawn from
research on primates (Sapolsky et al, 1997; Cohen et al, 1997). In this work, researchers have
documented a strong relationship between social status within a group and health outcomes.
This relationship has been found to persist even after experimental manipulation of a primate’s rank or status within a group, something that is impossible to do with humans.
A third and final reason why inequality and health might be related are the externalities associated with having richer neighbors. Depending on the nature of the externality, however,
having wealthy neighbors could be either beneficial or harmful to the less wealthy. Examples of
a positive externality include an increase in the availability of certain medical services that have
high fixed costs and as a result only locate in counties with high average levels of income
(assuming the services are normal goods). As a result, relatively poor individuals in the same county benefit from the increased availability of this service. A second example of a positive externality might be reductions in pollution - a public good that is also a normal good. An example of a negative externality is when an increase in average income in an area is associated with an increase in private expenditures on health and offsetting reduction in public expenditures on health, making the relatively poor worse off. Another potential negative externality would be an increase in segregation in unequal areas that can negatively affect access to goods and services and social capital among the poor (Durlauf, 1996).3
Of the three potential mechanisms by which inequality might be associated with health,
the first is relevant only in the context of aggregate data. Because we have individual data that
3 The policy implications of the first and the last two mechanisms differ. With respect to the first mechanism (heterogeneity and non-linearities in the production of health), increasing the wealth of the wealthiest individuals would not affect health. In contrast, for the latter two mechanisms, it would.
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can be aggregated, we can explore the extent to which non-linearities in the production of health
can explain the relationship between the Gini and average health observed in aggregate data.
With respect to the other two mechanisms, stress and externalities, we will be able to shed
some light on the relative importance of the latter by examining how inequality affects behavior
(investments), pollution levels (a public good), NICU availability (a service with high fixed costs) and segregation of the poor in separate hospitals.
C. Previous Evidence on Inequality, Relative Income and Health
The first evidence of a relationship between inequality and health was presented by Rodgers in 1979 who conducted cross-country analyses of the relationship between Gini coefficients,
GNP and multiple measures of health (infant mortality, life expectancy of birth) and found a strong relationship between both income and inequality and health. Kennedy (1996) provided evidence of a strong relationship between mortality and inequality across the 50 US states. Since then, many others have conducted within country, cross-area analyses of the relationship between inequality and health. These studies have been reviewed elsewhere (Subramanian and
Kawachi, 2004). In sum, evidence of a relationship between inequality and health is not conclusive but the estimated effects are strongest in the US and other more unequal countries, though there are also US based studies that find no relationship between inequality and health.
Nearly all of the existing analyses are based on cross sectional comparisons of adult mortality, which may be subject to considerable omitted variable bias. Indeed, Deaton and
Lubotsky (2002) show that for the cross-state and cross-MSA level analyses for the US, once one controls for the share of the local area that is black, the negative relationship between inequality
8 and average mortality disappears. This is due to the fact that inequality is higher in areas with a large share black and so is mortality (for both blacks and whites), though for reasons unknown.
As such, the share black is an important omitted variable in analyses of the relationship between inequality and average health.
There is less work examining the impact of relative deprivation. Evans and Eibner (2005) estimate the impact of relative deprivation on mortality from cardio-vascular disease using NHIS survey data linked with mortality data from the period 1988-1991. They define a reference group as those of the same race, age and education class in one’s own state of residence. They find that relative deprivation is predictive of higher mortality, worse self-reported death, higher
BMI and riskier behavior, controlling for own income.
Miller and Paxson (2006) also estimate the impact of relative income on mortality in an effort to explain black-white differentials in mortality. Specifically, they assess the extent to which blacks’ lower relative income can explain their higher rates of mortality. Using mortality data at the county level and census data on income at the puma (county-group) level, they find that within a county, the average income of blacks affects the black mortality rate, but that the average income of whites in the county also affects the black mortality rate. In fact, for black men, an increase in the average income of white residents of the same county increases the black mortality rate by about half as much as a similar-size decrease in black income.
II. Empirical Strategy and Data
A. Empirical Strategy
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Our analysis consists of two parts. In the first we estimate the impact of the Gini on newborn health (birth weight, low birth weight). We define one’s reference group (the population over which we construct the Gini) multiple ways: all households in the state, all new mothers in the state, all households in the county and all new mothers in the county. We begin with the most parsimonious regression based on aggregate data, add more controls and finally instrument for the Gini. The IV strategy exploits the variation in the growth of the Gini over this period that derives from differences in the initial (1970) distribution of income in a local area and national trends in income growth or returns to skill over this period. Specifically, to construct the instrument we follow Boustan et al (2012) and hold the income distribution of the county (or state) fixed at its 1970 level and predict changes in the distribution of income based on the initial income distribution in 1970 and national trends in income growth over this period. In this way, to borrow the language of Boustan et al (2012), we construct a “synthetic” version of the income distribution in each area that is not a function of the changing composition of the area. Note that we do not have an instrument for individual income. While this is consistent with existing empirical work on the topic (which doesn’t instrument for inequality either), we are considering ways to instrument for individual income as well.
We follow the aggregate analysis with an analysis of the impact of the Gini on newborn health using individual level data for 1970-1990 that include state identifiers (note: when we get the non-public NSFG data we will also conduct county level analyses). Controlling for individual income does not change any of the results.
The focus of the second part of the paper is an exploration of whether the above null effects may mask heterogeneity in the effect of inequality on health. To do so, we focus on the relationship between relative income and health using individual level data. We hypothesize that
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relative income may matter, with those at the top of the distribution benefitting and those at the
bottom suffering. To estimate this we construct measures of relative deprivation (YRD) initially
developed by Runciman (1966) and refined by Yitzhaki (1979). The deprivation measure is: